1. Field of the Invention
The present invention relates to a process for hydrogen-pulverizing a rare earth metal-based magnetic material for producing an alloy powder for a permanent magnet made from, for example, R (which is at least one of rare earth elements including Y), B and Fe as main components, and to a hydrogen pulverizing case used in the hydrogen pulverizing process.
2. Description of the Related Art
The conventional pulverization of such a type of a magnetic material is carried out in the following manner: An alloy ingot having a preselected composition corresponding to that of a rare earth metal-based magnet to be produced, is embrittled by occluding hydrogen (H.sub.2) thereinto in a hydrogen gas (H.sub.2) atmosphere by utilizing an H.sub.2 occluding property of the alloy ingot, thereby producing a coarse powder of rare earth metal-based magnetic material efficiently and in a short time.
In the conventional hydrogen pulverization of the rare earth metal-based magnetic material, however, the magnetic material is embrittled with occlusion of hydrogen, and risen in temperature up to 300.degree. C. to 350.degree. C. by an exothermic reaction during occlusion of hydrogen. Thereafter, to enhance the oxidation resistance of the magnetic material pulverized, it is necessary to further heat the magnetic material to 400.degree. C. to 600.degree. C. in vacuum or in an inert atmosphere to release a portion of occluded hydrogen. However, even if the hydrogen is released, the rare earth metal-based magnetic material is active and liable to be oxidized at a high temperature. For this reason, it is necessary to cool the magnetic material to a level on the order of 20.degree. C. to 30.degree. C. by an inert gas such as argon gas and then remove the cooled magnetic material into the atmosphere. Particularly, a rare earth metal-based magnetic material produced by a strip casting process which will be described hereinafter is in the form of flakes and hence, has a large surface after pulverization, as compared with a material produced in a mold-casting process. Unless the rare earth metal-based magnetic material is removed after being cooled reliably to the level on the order of 20.degree. C. to 30.degree. C., there is a possibility that a firing occurs. When the rare earth metal-based-magnetic material in the form of the flakes produced by the strip casting process is placed into a pulverizing container, the flakes are placed one onto another and for this reason, an inner part of the material in the container is more difficult to cool, as compared with a rare earth metal-based magnetic material in the form of an ingot produced by a mold-casting process. Therefore, in the hydrogen pulverizing process governed by the thermal treatment of such heating and cooling, it is a subject for enhancing the productivity to conduct the heating and cooling in a short time.